Existing beamforming technology using multiple antennas may be classified into analog beamforming technology and digital beamforming technology according to a location where a beamforming weight vector/precoding vector is applied.
First, an analog beamforming scheme is a representative beamforming scheme applied to an initial multi-antenna structure, which branches analog signals subjected to digital signal processing into a plurality of paths to perform beamforming through a phase shifter (PS) and a power amplifier (PA) of each path. FIG. 1 shows a conventional analog beamforming scheme.
Referring to FIG. 1, for analog beamforming, an analog signal derived from a single digital signal is processed by a PA and a PS connected to each antenna. That is, in an analog stage, a complex weight is processed by the PS and the PA. Here, a radio frequency (RF) chain means a processing block for converting a single digital signal into an analog signal.
However, in the analog beamforming scheme, beam accuracy is determined according to element characteristics of the PS and the PA. In addition, the analog beamforming scheme is suitable for narrowband transmission due to control characteristics of the element. In contrast, due to a hardware structure in which it is difficult to implement multiple stream transmission, multiplexing gain for increase in transfer rate is relatively small. In addition, it is difficult to perform beamforming per user based on orthogonal resource assignment.
Next, in the digital beamforming scheme, unlike the analog beamforming scheme, beamforming is performed in a digital stage using a baseband processor in order to maximize diversity and multiplexing gain in a multiple input multiple output (MIMO) environment.
Referring to FIG. 2, precoding is performed in a baseband processor such that digital beamforming is possible. Unlike FIG. 1, an RF chain includes a PA, because a complex weight derived for beamforming is directly applied to transmitted data.
In addition, since different beamforming is performed per user, it is possible to simultaneously support multi-user beamforming. Since beamforming is independently performed per user, to which orthogonal resources are assigned, scheduling flexibility is high and a transmitter suitable for a system purpose can be employed. In addition, if technology such as MIMO-orthogonal frequency division multiplexing (OFDM) is applied in a wideband transmission environment, it is possible to form an independent beam per subcarrier. Accordingly, the digital beamforming scheme can maximize a transfer rate of a single user based on enhanced beam gain and system capacity enhancement.
In a current 3G/4G system, digital beamforming based MIMO technology has been introduced according to above-described merits and demerits.